I don't know why anyone would want to do this, I am sure the only purpose it serves is to limit you from accessing content that is not licensed for international distribution. I don't think there is any exclusive DR content.
Actually this is good news, don't know how they do it, but there is obviously away, I guess by checking your IP address and seeing where it originates from. I know I for one about a month ago, tried to buy something from the itunes shop and told it was not available in this area! I have had that occurrence from many places over the net. Even for example if I go to a network TV webpage and try to watch something that I used to be able to watch, I am often told it is not available from my location.
So although it probably isn't exclusive DR content, it allows you to get content that was previously not available in the DR.